A fuel cell which draws a lot of attention as an alternative clean energy source is a generation system that converts the energy generated from electrochemical reaction between fuel and oxidant directly into electrical energy. With a rapid acceleration of environmental problems, depletion of energies and commercialization of fuel cell vehicles, various polymeric membranes applicable to high temperature are widely developing.
Fuel cells are generally classified into a solid oxide fuel cell operating at 700° C. or more, a molten carbonate electrolyte fuel cell operating at 500-700° C., a phosphate electrolyte fuel cell operating at about 200° C., and an alkaline electrolyte fuel cell and a polymer electrolyte fuel cell operating at room temperature to about 100° C. Among these fuel cells, a direct methanol fuel cell can be miniaturized since methanol need not be reformed.
Among those fuel cells, a polymer electrolyte fuel cell is environmentally friendly and, moreover, has a high power density and energy conversion efficiency. It is also possible to operate at room temperature, miniaturize and seal a polymer electrolyte fuel cell. Thus, this is widely applicable to no-pollution cars, home generation systems, mobile telecommunication equipment, medical devices, military equipment, aerospace equipment, etc. Consequently, researches are increasingly focused on this fuel cells.
Especially, a proton exchange membrane fuel cell (PEMFC) utilizing hydrogen gas fuel is a power generation system that produces DC electricity from an electrochemical reaction between hydrogen and oxygen, and has a structure where a proton conductive polymer membrane with thickness of 100 μm or less is inserted between an anode and a cathode. Therefore, a hydrogen molecule decomposes to a hydrogen ion and an electron by oxidation reaction at an anode as a reacting gas, hydrogen, is supplied. At this time, a reduction reaction that an oxygen molecule accepts electrons to become oxygen ions is occurred when the hydrogen ion is transferred to the cathode through the proton conductive polymer membrane. The oxygen ion generated reacts with the hydrogen ions transferred from the anode to become a water molecule.
In these procedures, the proton conductive polymer membrane is electrically isolated, but acts as a medium that transfers hydrogen ions from the anode to the cathode during cell operations and simultaneously separates fuel gas or liquids from oxidant gas. Thus, the membrane should have an excellent mechanical property, electrochemical stability and thermal stability at an operating temperature. In addition, it is required that the membrane can be fabricated as a thin film in order to reduce friction and should not expand much when containing liquid.
The conventional electrolytic membrane that has been widely used to polymer electrolyte fuel cells is Nafion developed by Du Pont. However, although the Nafion has a good proton conductivity (0.1 S/cm), it has disadvantages that its mechanical strength is poor such as a low tensile strength of 20 MPa and a water swelling of 40%. The price of the most commercialized fluorinated polymer, the Nafion, is about 100 $/cm2, whereas that of a typical hydrocarbon polymer is about 6-10 $/cm2. Thus, Over 10% of the total PEMFC MEA price can be curtailed by substituting Nafion with hydrocarbon polymer. However, since the degree of phase separation between hydrophobic main chain and hydrophilic side chain is lower than that of fluorinated polymer in spite of higher IEC of a hydrocarbon polymer than Nafion, the diameter of a ion cluster of the hydrocarbon polymer is 4-5 nm, which is 50% smaller than that of Nafion. Due to this small ion cluster, the ion conductivity of the hydrocarbon polymer is 0.05 S/cm, which is half of the Nafion having the ion conductivity of 0.1 S/cm. Accordingly, there are researches for improving the ion conductivity of the hydrocarbon polymer so as to be higher ion conductivity than the Nafion. However, although a typical hydrocarbon polymer, sulfonated polyetheretherketone (sPEEK), has a water swelling of below 20% owing to an aromatic main chain having a high stiffness, the water swelling of the sPEEK significantly increases and dissolves into water when the degree of sulfonation exceeds 75%.
The Korean Patent Registration No. 10-804195 discloses a high temperature-type hydrogen ion conductive polymer electrolyte membrane that has a high conductivity at high temperature by introducing a sulfonic group into a inorganic nanoparticle and, then, making a composite material with a polymer electrolyte. However, there is a disadvantage that the proton conductivity of this composite membrane is low since several tens to several hundreds nanometer-sized inorganic particles hinders the proton transport. In addition, the mechanical strength of the composite membrane is lowered owing to the size and aggregation of the inorganic particles.
The Korean Patent Application Publication No. 10-2013-118075 (‘Proton-conductive nanocomposite membrane utilizing silsesquioxane having a sulfonic acid group’) invented by the present inventors discloses a composite membrane comprising fluorine-based proton conductive polymer, such as Nafion, mixed with silsesquioxane. According to this document, the mechanical strength and conductivity of the electrolyte membrane is enhanced by using several nanometer-sized silsesquioxane and, however, there are still some disadvantages such as high production cost, a decrease of conductivity during long-term use, a rapid decrease in performance above 80° C., etc. In addition, there is still a need for a novel nanocomposite membrane electrolyte which has a higher ion conductivity in order for substitution of expensive Nafion.